Managing Invasive Grasses by Prescribed Herbivory Draft – February 15, 2006 James W. Bartolome, Craig Thomsen, and Sheila Barry (A condensed version of this paper was presented at the Society for Range Management meeting, February 15, 2006, Vancouver, Canada) 1. Introduction (JB) Prescription grazing is the application of livestock grazing prescriptions (i.e. specified grazing intensities, timings, frequencies, and degrees of selectivity) to purposely achieve natural resource objectives. Livestock performance is a secondary goal in prescribed livestock grazing. This paper discusses some general principles and examples of the use of prescribed herbivory to manage invasive grasses. Most of the prescription grazing work has been done with broad-leaved weeds. Although grasses are important components of rangeland ecosystems and important invasive weeds, relatively little work has been done on specific grass taxa. Grasses possess a suite of biological characteristics that may require a different approach to planning and understanding control. 2. Grass Biology (JB) Grasses probably arose in the southern hemisphere as Gondwanaland broke up in the late cretaceous. By the Miocene grassland ecosystems were widespread, the only biome dominated by a single plant family (Soderstrom et al 1987). Although several plant families have more species, none are as widespread (grasses are the only angiosperm family found on all seven continents) or as important for people. Most human food calories are consumed from the grasses maize, wheat, and rice. Grasses are a large (more than 7500 species) monophyletic family of monocots with a suite of important features reflecting common ancestry (Kellogg 2000). They have distinctive wind-pollinated or apomictic flowers and a modular growth form. Grass stems, termed culms, can best be thought of as an assembly of phytomers, which consist of a node, internode, sheath, and blade. The phytomer contains intercalary meristems at the nodes and base of the blade (Briske 1991). The myriad of variations on the phytomer theme is an important aspect of grass diversity and should allow development of some common principles related to prescribed grazing. The morphology of the phytomer and its intercalary meristems allows rapid replacement of aboveground parts removed by grazing or fire and hence grazing tolerance. Grasses have evolved C4 photosynthetic pathways independently several times, which increases the efficiency of energy capture at high temperatures and under conditions of low carbon dioxide concentrations, and improves water use efficiency. Grasses generally do not produce secondary toxic compounds and instead avoid and tolerate grazing through morphological mechanisms (Gould and Shaw 1983). Because grasses dominate extensive ecosystems, they are often linked to herbivore evolution and putative adaptive characteristics like hypsodontia, complex digestive systems, and forage preference. Grazers can be linked to grasses in an evolutionary context to greater degree than to other plant categories. The impact of the different grazing species on grasses and possibility for control should be defined in terms of intensity, frequency, distribution, and timing of defoliation (Heady and Child 1994). These effects will differ by target species. Grasses have been around a long time and form an important part of widespread natural ecosystems. They play roles ranging from dominant species in equilibrium – type communities to invaders and colonizers in highly disturbed and/or non-equilibrium – type systems. The distinction between equilibrium and non-equilibrium systems can be very important to the manager because the dynamics of non-equilibrium systems are primarily driven by abiotic factors. Grazing and its management have little lasting effect on these communities. “Exotic” is a political term normally contrasted to “native” or “indigenous” or “naturally occurring” that may be difficult to apply consistently. Often the term exotic is coupled with the term “invasive” to describe weedy problem species. These terms need careful definition and placement in the context of an appropriate model for vegetation dynamics. A weed is defined by its situation as perceived by the manager. 3. Grazers and grazing impacts (JB/CT) The primary producers in ecosystems fix energy, primarily in the form of carbon compounds, which is then transferred through trophic levels to support consumer organisms. The first transfer is to primary consumers (herbivores) through herbivory. This complex interaction is well-understood for many systems. When considering the effects of large herbivores on the system it is useful to separate the process of effects of herbivory into intensity, timing, frequency, and duration (Heady and Child 1994). Intensity is defined as the proportion of forage (herbage available for grazing or browsing) removed. It may be measured in a variety of ways, but for grasses it is usually determined by measuring the amount of above-ground phytomass either with or without grazing or before and after a grazing period. Because intensity is strongly linked to plant responses to defoliation and the ability to compensate for loss of herbage, it is a very important aspect of grazing management (Richards 1993). The timing, frequency, and duration of grazing also are important in determining the impacts of grazing on target plant species. These factors often interact with changes in animal preference and palatability that can become keys to developing grazing prescriptions. Animals affect rangeland systems apart from the direct impacts of herbivory (Stuth 1991). They redistribute nutrients, affect soil properties through physical impacts, trample uneaten forage, and affect water quality. They serve as important vectors for moving plants and other organisms. On the animal side, Walker (1995) presented some general grazing management research ideas that are relevant to invasive plant suppression, including grasses. He discusses how grazing will be conducted on extensive rangelands in the future, stating that successful management will be based on the ability to accomplish three objectives: 1) control what animals graze, 2) control where they graze, and 3) monitoring the impact on both the environment and the animal. Among Walker’s research suggestions are genetic manipulation of livestock to select diets that are most appropriate for the environment and management goals of the grazier including: 1) classical selection and animal breeding techniques, 2) inserting a specific gene into a breed, i.e., a sheep gene that can handle Senecio jacobaea might be used in cattle since sheep have an enzymatic system that allows them to detoxify this species, 3) Genetically modifying microbial symbionts for ruminants, tailored to certain foraging environments that would allow them to digest certain materials. Some of this is already being used or tested. 5. Species list of important North American invasive grasses (adapted primarily from www.fs.fed.us/database/feis) Aegilops cylindricus (goatgrass) Aegilops triuncialis (barbed goatgrass) Agropyron desertorum (desert wheatgrass) Agropyron cristatum (crested wheatgrass) Arundo donax (giant reed) Brachypodium distachyon (false brome) Bromus hordeacous (soft chess) Bromus inermis (smooth brome) Bromus japonicus (Japanese brome) Bromus tectorum (cheatgrass) Cynodon dactylon (Bermuda grass) Dactylis glomerata (orchard grass) Echinochloa crus-gali (barnyard grass) Eragrostis curvula (weeping lovegrass) Eragrostis lehmianii (Lehman lovegrass) Elytrigia repens (quackgrass) Festuca arundinacea (tall fescue) Pennisetum setaceum (fountain grass) Phalaris arundinacea (reed canarygrass) Phalaris aquatica (Harding grass) Poa pratensis (Kentucky bluegrass) Sorghum halepense (Johnson grass) Spartina alterniflora (European beachgrass) Taeniatherum caput-medusae (medusahead) Below is a template for evaluating the possible role of prescription grazing in control of common grass species. The table lists key characteristics necessary for designing a management approach. This information will only rarely be available and it will usually be necessary to use an adaptive management approach with close monitoring of plant responses. Invasive Where a Growth Reproductive Invasiveness Seed bank Seed Season & Frequency Intensity Kind & Stock Demonstrated Citation grass problem form strategy longevity volume growth class of density successful species stage for livestock control with grazing P.G. Bromus Rangeland annual Seed high 1 year high EV, Twice high C & S high yes Moseley tectorum MV, S 1999 Aegilops Rangeland annual Seed High 2+ medium LV, B single high C high no Betts triuncialis 2002 Spartina alterniflora Arundo donax Agropyron desertorum Crop, irrigated pasture, riparian, rangeland A or P, WS or CS, SF or BG V or S (Vegetative or Seed) H, M or L (High, Medium or Low) 0, 1, or 2+ years W, Sp, S or F (Winter, Spring, Summer or Fall); EV, MV, LV, B, F, S (Early-vegetative, Mid-vegetative, Late-vegetative, Boot, Flower, Senescence) Year and within year S and C (Sheep and cattle) 6. Examples (successes) (CT) Relatively few research papers report the effective use of prescription grazing to control the invasive grass species on the above list. Perhaps the best documentation reports Bromus tectorum (cheatgrass) reduction from livestock grazing, especially sheep. Cheatgrass is an annual grass introduced to the Intermountain region from Eurasia. It spread rapidly onto rangelands in the early 20th century and now dominates many millions